1. Field of the Invention
The present relates to an access point, wireless communication station, wireless communication system, and wireless communication method for performing wireless communication based on a packet transmission system which allocates wireless bands by CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance).
2. Related Background Art
Recently wireless LAN interfaces are installed in various devices. In particular, the popularization of the IEEE 802.11 standard for personal computers, home electronic products and mobile terminals is remarkable. When a mobile terminal is used as a wireless communication station based on the IEEE 802.11 standard, the critical technology deals with power saving. Document “ANSI/IEEE std 802.11, Wireless LAN medium access control (MAC) and physical layer (PHY) specifications, 1999” describes the technology to implement power saving in wireless communication stations, using the following method.
In the case of the wireless LAN communication specified in IEEE 802.11, based on the cycle of common notification information (hereafter called “beacon”), an access point transmits beacons to wireless communication stations. A beacon includes a bit map called “TIM” (Traffic Indication Map), which indicates the presence of data stored for each wireless communication station which is currently operating in power saving mode. According to the Listen Interval (parameter to specify the cycle of receiving beacons), the wireless communication station, which is operating in power saving mode, regularly transits from the reception stopping state (hereafter “Doze state”) to the reception standby state (hereafter “Awake state”), so as to receive beacons. The wireless communication station judges the presence of data addressed to this wireless communication station in the access point, by analyzing the TIM information element (bit map) of the received beacon. If data addressed to this wireless communication station is stored, this wireless communication station transmits a control frame called “PS-Poll” (Power Save-Poll), so that data is transferred from the access point to this wireless communication station, and the Awake state is maintained until the last frame is received. If the data addressed to this wireless communication station is not stored, on the other hand, this wireless communication station transits to the Doze state. In other words, the wireless communication station in power saving mode repeats transitions between the Awake state and the Doze state at a predetermined cycle.
In the above mentioned prior art, the power saving effect can be expected for the wireless communication station because of the intermittent reception, but the access point must maintain the Awake state at all times. There is no problem if the access point is in a state of constantly receiving a power supply, but if a mobile wireless communication device (that is, a wireless communication device which is not always in a state of constantly receiving a power supply) plays a role of an access point, power consumption of the access point becomes critical in the case of the above mentioned prior art.